This invention relates to adsorption apparatus and methods of gas adsorption.
Separations of gases have long been important in many industrial processes. Removal of carbon dioxide continues to be an important objective for purifying air for humans to live underwater and in space. Other important technologies that can utilize improvements for gas separation include: fuel cells, ammonia production, fertilizer manufacture, oil refining, synthetic fuels production, natural gas sweetening, oil recovery and steel welding.
The adsorption capacity of a gaseous species onto an adsorbent is commonly expressed in graphical form in adsorption isotherms and isobars, which are widely published in the literature and by adsorbent manufacturers and suppliers. For the sorption of gas species, the capacity is typically expressed as the equilibrium mass of the species sorbed per unit mass of adsorbent (e.g., kg species/100-kg adsorbent). The sorbent capacity varies as a function of temperature and the partial pressure (concentration) of the species being sorbed. Loading or capacity typically increases as the adsorbent bed temperature decreases or the partial pressure of the sorbed species in the gas phase increases.
The variation of adsorption capacity with temperature and pressure can be used to effect separations of gas species. For example, in pressure swing adsorption (PSA) gas species are adsorbed onto a sorbent at relatively high pressure, tending to remove the species from the feed stream. In a regenerative PSA process, reducing the absolute pressure (e.g., applying a vacuum) to the loaded sorbent bed or reducing the partial pressure of the sorbed species in the gas phase by sweeping a lower concentration purge gas through the bed regenerates the sorbent. Cycle times for PSA processes are typically measured in minutes (Humphrey and Keller, xe2x80x9cSeparation Process Technology, McGraw-Hill, 1997). In a regenerative temperature swing or thermal swing (TSA) adsorption process, species are adsorbed at low temperature where the loading capacity is relatively high and (at least partially) desorbed at higher temperature, thus recovering sorption capacity for additional cycles.
In addition to gas species separations, TSA can be used to thermochemically compress gases. Sorption based thermochemical compression is applicable to refrigeration and heat pump cycles (e.g., see Sywulka, U.S. Pat. No. 5,419,156) and for chemical processing in general.
Gas adsorption is known to be applicable to a wide range of gas species (see, e.g., Kohl and Nielsen, Gas Purification, 5th Ed., Gulf Publ. Co., Houston, Tex.). Kohl and Nielsen report that in conventional TSA gas purification processes, adsorbent bed loading and unloading cycles are typically on the order of hours.
Despite their long-known use and importance, multiple problems remain with gas adsorption separation technologies. These problems include: use of excess energy, bulky apparatus or low capacity, cost, and slow rate and/or low mass of gas separated.
In one aspect, the present invention provides a sorption pump that includes an adsorption layer comprising an adsorption mesochannel containing adsorption media, and a heat exchanger in thermal contact with the adsorption layer. The heat exchanger includes at least one microchannel. The adsorption layer has a gas inlet such that gas directly contacts the adsorption media without first passing through a contactor material.
In another aspect, the invention provides gas adsorption and desorption apparatus that includes at least one adsorption layer comprising an adsorption mesochannel containing adsorption media. The adsorption mesochannel has dimensions of length, width and height; wherein the height is at least 1.2 mm. The apparatus possesses capability such that, if the adsorption media is replaced with an equal volume of 13xc3x97zeolite, having a bulk density of about 0.67 grams per cubic centimeter, and then saturated with carbon dioxide at 760 mm Hg and 5xc2x0 C. and then heated to no more than 90xc2x0 C., at 760 mm Hg, then at least 0.015 g CO2 per mL of apparatus is desorbed within 1 minute of the onset of heating. By heated to xe2x80x9cno more than 90xc2x0 C.xe2x80x9d typically means that 90xc2x0 C. water is passed through the heat exchanger; however, the phrase also encompasses heating by other means such as an electrically-resistive heater. Preferably, the apparatus includes at least one heat exchanger in thermal contact with the adsorption layer. In preferred embodiments, the apparatus possesses capability such that, if the adsorption media is replaced with an equal volume of 13x zeolite, having a bulk density of about 0.67 grams per cubic centimeter, and then exposed to carbon dioxide at 760 mm Hg and 5xc2x0 C. for 1 minute and then heated to no more than 90xc2x0 C., at 760 mm Hg, then at least 0.015 g CO2 per mL of apparatus is desorbed within 1 minute of the onset of heating.
In yet another aspect, the apparatus is configured to selectively heat the adsorbent. By xe2x80x9cselectivelyxe2x80x9d it is meant that the apparatus is configured to heat the adsorbent material in preference to other parts of the apparatus; more particularly, where the adsorbent occupies only a portion of the adsorbent layer, heat is added to the adsorbent in preference to other parts of the adsorbent layer. For example, the at least one heat exchanger could be configured such that the heat exchange fluid flow paths substantially overlap the area of adsorption channel or channels. Alternatively, the apparatus could contain a relatively thermally conductive material overlapping the adsorption channel or adsorption channels and a relatively thermally insulating material that does not substantially overlap the adsorbent channel or adsorbent material. By xe2x80x9csubstantially overlapxe2x80x9d it is meant that, when viewed from a direction perpendicular to the direction of flow in which the adsorption channel and heat exchanger is stacked, the areas of the adsorbent channel(s) and the thermally conductive material have at least about an 80% overlap.
In a further aspect, the invention provides a sorption pump, that includes an adsorption layer comprising an adsorption channel containing adsorption media, and a mesochannel heat exchanger in thermal contact with the adsorption layer. The mesochannel heat exchanger has a fluid flowing therethrough that has a high thermal diffusivity, such that the characteristic heat transport time for the fluid in combination with the mesochannel heat exchanger is no greater than 10 seconds.
The invention also provides an apparatus in which adsorption/desorption cells are connected to improve overall energy efficiency. Each cell contains at least one adsorption mesochannel having an inlet and/or outlet. Typically, each cell contains multiple adsorption mesochannels that share a common header and common footer, and that are operated together. Preferably, each adsorption channel is in thermal contact with at least one heat exchanger. Each adsorption channel contains adsorption media. Typically, the apparatus also contains or is used in conjunction with a heat source and a heat sink. In some embodiments, the heat sink could be the non-adsorbed gas, which is passed through and removed from the apparatus. The apparatus contains heat transfer conduits between each cell and the heat source and heat sink and also contains heat transfer conduits between each cell and at least two other cells. In operation, the conduits carry a heat exchange fluid or can contain a thermally conductive material. The apparatus also contains valves that can control gas flow into the at least one adsorption channel. Cell volume is defined as the volume of the adsorption channel or channels that are operated together, including the volume of the heat exchange channel or channels, the volume between such channels, the volume of the outer walls of the cells, and the volume of inlet and outlet footers, when present.
The invention further provides a method of gas adsorption and desorption, comprising passing a gas into an adsorption layer where at least a portion of the gas is adsorbed onto adsorption media to form an adsorbed gas and selectively removing heat from the adsorption layer through a distance of 1 cm, preferably 2 mm, or less into a heat exchanger; and, subsequently, selectively heating the adsorption media through a distance of 1 cm, preferably 2 mm, or less from a heat exchanger, and desorbing gas. The gas directly contacts the adsorption media without first passing through a contactor material. For more rapid heat transfer (and thus faster cycling), the adsorption channel may contain heat transfer agents such as metal fins or pins, or graphite fibers.
The invention also provides a method of gas adsorption and desorption, comprising passing a gas into an adsorption layer where at least a portion of the gas is adsorbed onto adsorption media to form an adsorbed gas and selectively removing heat from the adsorption layer through a distance of 1 cm or less into a heat exchanger; and, subsequently, selectively heating the adsorption media through a distance of 1 cm or less from a heat exchanger, and desorbing gas. In some cases, distances are specified as between the center line of an adsorption layer and the center line of a heat exchangerxe2x80x94the center lines are apparent from the figures or can be ascertained in any laminated (layered) device; alternatively, this limitation can be replaced by a limitation of equal length that any point in the adsorbent is within a stated distance (typically 1 cm) of a heat exchanger.
The invention further provides a method of gas adsorption and desorption in a gas adsorption and desorption apparatus comprising at least one adsorption mesochannel and at least one heat exchanger. In this method, gas is adsorbed into adsorption media in at least one adsorption mesochannel and, simultaneously, heat is removed from the adsorption media into a heat-absorbing heat exchanger. Subsequently, heat is added from a heat-supplying heat exchanger to the adsorption media in the at least one adsorption mesochannel and gas is desorbed from the adsorption media. The combined steps of adsorbing a gas and desorbing a gas form a complete cycle; and, in a complete cycle, at least 0.1 mol of gas per minute per liter of apparatus is adsorbed and desorbed. The apparatus may include multiple adsorption mesochannels (for example, 2, 10 or more) and multiple heat exchangers. When multiple mesochannels or heat exchangers are employed, they may be of different types. For example, the heat-supplying heat exchanger could be an electrical resistance heater and the heat-absorbing heat exchanger could be an element of a thermoelectric cooler. Alternatively, the heat-supplying heat exchanger and the heat-absorbing heat exchanger could be the same channel (preferably, as discussed elsewhere, the heat exchanger contains microchannels) through which cold then hot fluid is passed. In some embodiments of this method, the temperature difference of the adsorbent media between adsorbing and desorbing is preferably less than 200xc2x0 C., more preferably less than 100xc2x0 C., and in some embodiments the temperature difference is between 50 and 200xc2x0 C. In some embodiments of this method, the cycle time is preferably less than 5 minutes, more preferably less than 2 minutes, and in some embodiments between 0.5 and 10 minutes. In some embodiments of this method, from 0.1 mol to 1 mol of gas per minute per liter of apparatus is adsorbed and desorbed in a complete cycle. This method may include any of the features or conditions described in this specification, for example, any of the device features could be employed.
The invention also provides a multi-cell sorption pump, comprising: at least six sorption cells; where each sorption cell comprises at least one adsorption layer, and at least one heat exchanger layer. Thermal connections connect each sorption cell to at least two other sorption cells and to a heat source and to a heat sink, such that each sorption cell can cycle thermally from adsorption to desorption and back to adsorption by sequentially receiving heat from said at least two other sorption cells prior to receiving heat from the heat source, and then sequentially giving up heat to at least two other sorption cells prior to giving up heat to the heat sink, such that thermal recuperation is provided.
The invention also provides a method of gas adsorption and desorption, comprising a first step of passing a gas into a first adsorption layer containing a first adsorption media where at least a portion of the gas is adsorbed onto the adsorption media to form an adsorbed gas and removing heat from the adsorption layer through a distance of 1 cm or less into a first heat exchanger. Subsequently, in a second step, the adsorption media is heated through a distance of 1 cm or less from the first heat exchanger, and gas is desorbed. Simultaneous with the first step, a heat exchange fluid flows through the heat exchanger and exchanges heat with the adsorbent. This heat exchange fluid flows into a second heat exchanger that, in turn, exchanges heat with a second adsorption layer containing a second adsorption media.
The invention also provides a method of gas adsorption and desorption that includes: a first step of transferring heat from a heat source into at least two first cells and desorbing gas from each of the two first cells, and transferring heat from at least two second cells to at least two third cells; a second step of transferring heat from the at least two second cells to a heat sink, and adsorbing gas into the at least two second cells, transferring heat from the at least two first cells to the at least two third cells; a third step of transferring heat from a heat source into the at least two third cells, and desorbing gas from each of the at least two third cells, transferring heat from the at least two first cells to the at least two second cells; and a fourth step of transferring heat from the at least two first cells to a heat sink, and adsorbing gas into the at least two first cells, transferring heat from the at least two third cells to the at least two second cells. In this method, each cell comprises at least one sorbent, and at least one heat exchanger.
The invention also provides a method of adsorption and desorption that provides the thermal enhancement of PSA adsorption, thereby obtaining greater utilization of the adsorbent media than would be accomplished by PSA adsorption alone. This includes cooling of the adsorbent media during adsorption at one partial pressure of the adsorbing specie(s), so that a greater amount of adsorbing specie(s) can be adsorbed, and/or heating of the adsorbent media during desorption at a lower partial pressure of the desorbing specie(s), so that a greater amount of desorbing specie(s) can be desorbed. In general, the methods described herein are applicable for thermal swing adsorption, thermally-enhanced pressure swing adsorption, and thermochemical compression.
In another aspect, the invention provides a sorption pump comprising: an adsorption layer comprising an adsorption mesochannel containing adsorption media; and a heat exchanger layer adjacent the adsorption layer, the heat exchanger layer comprising a first region comprising a first heat exchange fluid pathway and a second region comprising a second heat exchange fluid pathway. The first fluid pathway connects a header and a footer. The second fluid pathway also connects a header and a footer. The first fluid pathway has a shorter average length than the second fluid pathway where length is measured in the direction of net fluid flow through the heat exchanger layer. The product of the average width and average height (widthxc3x97height) of the second fluid pathway is larger than the product of the average width and average height (widthxc3x97height) of the first fluid pathway. In a broader aspect, the invention provides the above-described heat exchanger by itself or in combination with any layers in need of heat exchange.
The term xe2x80x9cregionxe2x80x9d refers to a selected contiguous volume in the heat exchanger layer. The region may be arbitrarily selected, but should include two sides that are parallel to length of a fluid pathway located therein. The height of a xe2x80x9cregionxe2x80x9d is typically determined by the height of the heat exchanger layer. The height of a heat exchanger layer in an unbonded stack is the height of the heat exchanger shim or shims, while in a bonded stack or integral device is the maximum height of a heat exchange fluid pathway. A xe2x80x9cfluid pathwayxe2x80x9d is a channel, open space, or any area that permits flow of a heat exchange fluid.
The invention further provide methods of adsorbing and desorbing a gas in the any of the sorption pumps described above, comprising: adsorbing a gas onto an adsorbent in the adsorbent mesochannel to form an adsorbed gas at a first temperature; passing a heat exchange fluid into the first and the second fluid pathways, wherein the heat exchange fluid is at a temperature that is higher than the first temperature; and desorbing at least a portion of the adsorbed gas.
In another aspect, the invention provides an integrated, multicell sorption pump, comprising: at least 3 cells disposed around a central axis, each cell comprising at least one unit, where each unit comprises a heat exchange layer and an adsorbent layer that is adjacent to the heat exchange layer; wherein the layers are substantially planar with mutually perpendicular dimensions of width, height and length, wherein length is measured in the direction of net fluid flow through each layer and wherein the height of each layer is smaller than its width and smaller than its length and wherein height of each layer is substantially parallel to the central axis.
While the various aspects of the present invention are generally applicable to any gas, including hydrogen, there are some preferred embodiments that are specifically directed to hydrogen. For example, hydrogen can be adsorbed into metal hydrides (metal hydride adsorbents can be initially supplied in the hydride form, or, more typically, as the metal that is converted to a hydride in situ) and rapidly desorbed using the apparatus and methods described herein. In a particular aspect, the invention provides a method of starting up a fuel cell that includes the steps of adsorbing and desorbing hydrogen. In this aspect, the invention provides a method of starting a fuel cell, comprising: (a) producing hydrogen from a reformer and adsorbing a portion of the hydrogen produced by the reformer to a hydrogen sorbent that is disposed in a mesochannel within a sorption pump (b) in the sorption pump, heating the hydrogen sorbent that is disposed in a mesochannel, causing hydrogen to be desorbed; and passing at least a portion of the desorbed hydrogen into the non-operating fuel cell; and (c) using the desorbed hydrogen to start the fuel cell.
Generally, the invention provides any of the components (for example, individual shims or collections of shims) or devices (for example, stacked shims, stacked shims with inlets and/or outlets and/or connecting fluid conduits, etc.) that are described herein. While various embodiments are illustrated as a series of shims, it should be recognized that the invention need not be characterized by shims but could be characterized (and claimed) by a device having certain connections, passages, and/or adsorbent configurations, etc. The devices and components can additionally, or alternatively, be characterized (and claimed) by their functions and/or their properties (for example, by their efficiencies) such as, but not limited to the properties evidenced by testing results. The invention also includes devices such as sorption pumps, heat exchangers, and gas adsorption and desorption apparatus. In some embodiments, these devices may be characterized in conjunction with their functions.
The invention also includes processes of using any of the components or devices (or any portion of the components or devices described herein). The invention also includes methods of gas adsorption and/or desorption and/or methods of exchanging heat. In some embodiments, the inventive methods may be characterized in conjunction with properties or other characteristics that are described herein or that result from the described devices or other descriptions herein.
The invention includes groups of shims that have been bonded into a laminated device or stacked shims prior to bonding as a stacked preassembly.
The invention further provides methods of making components or devices by steps including stacking shims having one or more of the features described herein. Following stacking the shims are typically bonded by a process such as diffusion bonding or (for low temperature applications) adhesives.
In a report (xe2x80x9cMicroscale Adsorption for Energy and Chemical Systemsxe2x80x9d) appearing on the PNNL web site in May 2000, Viswanathan, Wegeng and Drost reported the results of calculations and experiments for investigations of microchannel adsorption with short cycle times. From the reported estimate that 95% of CO2 reaches the zeolite particles in 30 seconds, based on semi-infinite diffusion, it is clear that this calculation involves zeolite adsorbent in a xe2x80x9cflow-byxe2x80x9d arrangement, rather than a xe2x80x9cflow-throughxe2x80x9d arrangement. A xe2x80x9cflow-byxe2x80x9d arrangement is one in which adsorbent occupies less than the full cross-sectional area of the flow path and gas flow is primarily by an adsorbent, requiring that contact of the adsorbent media, with the specie(s) to be adsorbed, occur primarily by mass diffusion into and through the adsorbent structure, while in a xe2x80x9cflow-throughxe2x80x9d arrangement the adsorbent is substantially placed within the flow path, so that the fluid flows directly xe2x80x9cthroughxe2x80x9d rather than adjacent to the adsorbent structure.
Various embodiments of the invention can provide numerous advantages including one or more of the following: rapid cycling, rapid sorbent regeneration, reduced time and/or larger volumes of gas adsorbed as a function of sorbent mass required, excellent device stability, low cost, direct sorption into the sorption media without requiring diffusion through a contactor, preferential heating/cooling of the sorption media to a greater degree than other elements of the adsorber structure, configurations of sorption units with recuperative heat exchange thereby allowing energetically efficient temperature swing separations and/or more energetically-efficient, thermochemical compression.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.
In the present invention, the term xe2x80x9cmicrochannelxe2x80x9d refers to a channel with at least one dimension, of 1 mm or less, preferably in a direction perpendicular to net flow through the channel. The term xe2x80x9cmesochannelxe2x80x9d refers to a channel with at least one dimension, in a direction perpendicular to net flow through the channel, of 1 cm or less. For both, the optimum design usually includes orienting the height of the channel in the direction for which rapid heat and/or mass transport is desired. An xe2x80x9cadsorption layerxe2x80x9d of a flow-through device includes only the adsorbent, but in a laminated flow-by device, the xe2x80x9cadsorption layerxe2x80x9d includes the adsorbent plus contactor (if present) plus the open area adjacent the adsorbent and/or contactor.
The xe2x80x9ctheoretical capacityxe2x80x9d of an amount of adsorbent is determined by maintaining the adsorbent at a first temperature, at a fixed partial pressure for the gas specie(s) to be adsorbed, for a sufficient period of time so that essentially no more gas will be adsorbed, then shutting off the gas flow and heating to a second temperature to desorb gas, at the same or another fixed partial pressure for the gas specie(s), until essentially no more gas is desorbed, and measuring the amount of gas desorbed; the amount of gas desorbed is defined to be the xe2x80x9ctheoretical capacityxe2x80x9d of an adsorbent material for that set of process conditions. The actual xe2x80x9ccapacity utilizedxe2x80x9d within a working sorption pump is measured at the same pressure and temperature conditions, but for a selected, finite period of time, and therefore may be less than the theoretical capacity.
A sorption pump is defined as a device which captures a gas, or constituents within a gas, onto the surface of an adsorbent media, and then desorbs at least a portion of the captured gas, as the system is brought to a different temperature and/or pressure. A sorption pump makes use of the change in equilibrium sorption capacity of a sorbent that occurs when temperature and/or pressure conditions are changed. A mesochannel sorption pump contains adsorbent material within a mesochannel, which is in thermal contact with a heat exchanger, preferably a microchannel heat exchanger, thereby providing rapid heat transfer between the adsorbent mesochannel and the heat exchanger microchannel.
Transport phenomena within microchannels and mesochannels generally exhibit characteristic heat and mass transport times between milliseconds and tens of seconds. Systems of microchannels and mesochannels, in combination with appropriately chosen heat transfer fluids, can therefore be designed that exhibit transient heat and mass transport response rates on the order of tens of seconds, or seconds, or faster, and mesochannel sorption pumps should therefore be able to operate through the complete TSA cycle within a few minutes or in some cases, within tens of seconds or less.
For aspects of the invention that are described in terms of hardware volume or apparatus volume, the volume includes adsorption media, heat exchangers, endplates and headers and footers, but does not include external conduits. For example, the apparatus would include the entirety of the apparatus shown in FIG. 15 except for the external conduits.